FIG. 4 is a schematic view showing a conventional method of production of electric resistance welded steel pipe. As shown in FIG. 4, in the conventional method of production of electric resistance welded steel pipe, generally strip shaped steel plate 101 is continuously conveyed toward a direction 110 while being shaped into a tube by a large number of rolls (not shown), the abutting end faces 104 are melted by induction heating by a high frequency coil 102 or by direct conduction heating by a contact tip, and squeeze rolls 103 are used to apply upset so as to form a weld seam 105 at the abutting end faces 104 and thereby obtain electric resistance welded steel pipe.
In such a conventional process of production of electric resistance welded steel pipe, the abutting end faces 104 are exposed to the atmosphere at the time of electric resistance welding, so oxides are formed at the surfaces. These sometimes remain without being squeezed out, whereby weld defects caused by the oxides, called “penetrators”, are formed at the weld zone. In particular, when using Cr steel with a Cr content of 2 to 11 mass %, stainless steel with a Cr content of 12 mass % or more, ferrite-martensite composite structure steel (DP steel; volume percentage of second phase martensite of 5% or more) and ferrite-austenite composite structure steel (TRIP type composite structure steel; low alloy high strength steel utilizing transformation induced plasticity of volume percent 5% or more residual austenite), or other such steel plate containing large amounts of Cr, Si, or other elements easily forming oxides, penetrators easily are formed in the weld zone. Such weld defects become causes lowering the low temperature toughness, corrosion resistance, and cold workability of steel pipe, so in the past, in the process of production of electric resistance welded steel pipe, an inert gas shield was used at the time of electric resistance welding so as to reduce the amount of oxygen in the atmosphere of the weld zone so as to try to reduce the formation of penetrators. However, with an inert gas shield, since air entrainment etc. occur, it is difficult to stably make the atmosphere of the electric resistance weld zone a low oxygen state. On the other hand, to stably maintain the low oxygen state of the electric resistance weld zone, a bulky shield apparatus would be necessary and the productivity would greatly drop.
Further, the inventors proposed a method of production of electric resistance welded steel pipe comprising blowing a 1400° C. or higher reducing high temperature combustion flame or nonoxidizing high temperature plasma on to the abutting end faces by a predetermined flow rate at the time of electric resistance welding steel pipe so as to suppress the formation of oxides at the abutting end faces and promote the expulsion of oxides (see Japanese Patent Publication (A) No. 2004-298961). The art described in this Japanese Patent Publication (A) No. 2004-298961 enables reduction of the penetrators in the electric resistance weld zone compared with the conventional method without lowering the productivity. However, this method has the problem that if trying to further reduce the penetrators at the weld zone by increasing the flow rate of the high temperature combustion flame or plasma to raise the shear force of the thermal fluid, the entrainment of air around the weld zone becomes remarkable and conversely the penetrators end up increasing. Therefore, with the art described in Japanese Patent Publication (A) No. 2004-298961, it is not possible to sufficiently reduce the formation of penetrators.
Therefore, the inventors engaged in further studies and proposed a method of production of electric resistance welded steel pipe comprising blowing inert gas at a predetermined butt angle on to the abutted end faces across the entire range from at least the weld point to the welding upstream side where the temperature becomes 650° C. or more and blowing a nonoxidizing high temperature plasma in a nonoxidizing atmosphere at a temperature of 1400° C. or more by a flow rate of 30 to 270 m/sec on to the abutted end faces across the entire range from at least the weld point to a position at the welding upstream side exactly ⅕ the power feed distance (power feed distance: distance from high frequency coil or power feed tip to weld point) (see Japanese Patent Publication (A) No. 2006-026691). With the method of production of electric resistance welded steel pipe described in this Japanese Patent Publication (A) No. 2006-026691, it is possible to reduce the weld defects caused by the formation of oxides at the weld zone and cold weld defects caused by insufficient local input heat without lowering the productivity and further without changing the set conditions at the time of fluctuation of the welding conditions.
Further, an invention relating to a plasma torch defining the side gas ejection direction so as to raise the processing efficiency and precision of high temperature working such as plasma jet welding, cutting, flame spraying, and heating is disclosed in Japanese Patent Publication (A) No. 2004-243374.